Electromagnetic flowmeters, as is well known, measure the volumetric flow rate of electrically conductive liquids. The measurement is based on Faraday+s law of induction. The liquid is passed through a magnetic field, so that a voltage is induced in it, which is taken off by means of at least two electrode assemblies.
The means for conducting the liquid in the form of a measuring tube, the means for generating the magnetic field, the electrode assemblies, and possibly further components form the electromagnetic flow sensor.
The inner surface of the measuring tube must, at least in the region of the magnetic field, be made of an insulating material in order that the induced voltage will not be short-circuited. Therefore, metallic measuring tubes are lined with a suitable insulating material.
Furthermore, metallic measuring tubes must not be made of ferromagnetic material, so that the magnetic field generated by means of a coil assembly disposed outside the measuring tube can reach and pass through the liquid. They are therefore generally made from high-grade steel, so that their external surface is stainless and generally requires no further treatment.
An electrode assembly comprises a measuring electrode with a head and a shank. The measuring electrode is fixed in a hole of the wall of the measuring tube in such a way that it is electrically insulated from the wall. This insulation must be very good, since the induced voltage has only small values of the order of 1 mV and its internal resistance may be on the order of 100 k.OMEGA..
The measure of the quality of the insulation is usually the insulation resistance between the measuring electrode and the measuring tube. In dry conditions, the insulation resistance is usually of the order of 100 .OMEGA..
It has turned out that during operation of the flow sensors, these values may deteriorate to the order of the internal resistance of the induced voltage, whereby the latter is reduced to approximately one half. Investigations by the inventors have shown that this occurs mainly when the measuring tube is colder than the ambient air. The ambient air usually leaks into a housing that encloses the electrode assemblies, even if the housing is filled with foam.
Thus, moisture will settle on the measuring tube, especially near the measuring electrodes; this may even result in the formation of water droplets. Due to capillary forces, the moisture will creep into gaps that may exist, on the one hand, between the measuring electrodes and the insulation and, on the other hand, between the insulation and the measuring tube. Furthermore, as a result of adhesion, films of moisture may form on the external surfaces of the insulation and the measuring electrodes.